The Athenians may not have sought an empire but could not let it go. How accurate is this statement and to what extent is Athenian imperialism a cause of the war
You have a couple of issues to deal with:
The Athenians may not have sought an empire
I'm not sure that's true. The sinking of the irons is a very powerful religious symbol (representing the permanance of the bond). Why would they have used that powerful symbol for the removal of the Persians, which happened so quickly? (I know, the joys of hindsight, but it's still a point that can be made)
The 'treasurers of the Greeks' were all Athenian from the start (Athens had control of the money and ships, even though they were held in Delos), and Athens could always use its position as hegemon, as a powerful city, and as the symbolic ancestor of the Ionians to bully other cities into voting their way.
The Athenians could not let go of their empire
This is certainly true. After the inevitable collapse of the Hellenic League (the poleis- plural of polis- were independant institutions and had almost always been fighting: it was the way things were done), relationships were tough between Athens (just newly a powerful city) and Sparta (which had been a powerful city from well within the Archaic Age), especially since at the turn of the 6th century Sparta tried to take over Athens (the Athenian revolution, you don't need to know about that- but that's the reason why the Athenians gave earth and water to the Persians in the first place, and one of the reasons for the Persian Wars).
The money was also very important for the Athenians in rebuilding their city (after the Persians burnt it to the ground), and for the upkeep of sailors (who were paid, unlike hoplites, who fought because of the honour attributed to it. Although wealthy Athenian citizens also kept up the triremes, the funding from the League/ Empire was necessary).
The Spartans were growing increasingly jealous of the Athenians, especially when Argos (voluntarily) and Megara (by force- which compromised the Corinthians- a reason why the Spartans became more involved) became a part of the Empire. The safety of Athens depended on this power (but ultimately they lost the Pel. war because of this), which is one reason why they took over Aegina.
To what extent was Athenian imperialism a cause of the war?
If you had told me which war that may have helped, but I'm assuming Peloponnesian. I haven't studied the Peloponnesian War, but take what I've said so far and combine it with what you know. The source I'd recommend would be Thucydides (and Bury and Meiggs for a modern source, or The Ancient Greeks by Fine et. al.) Hope I've been helpful!
Good luck!